The present invention relates to systems and methods for distributing radiation among a plurality of optical channels, and more particularly, to such systems and methods that more efficiently distribute light among optical channels containing non-linear optical elements.
In many modern optical systems, such as fiber-based telecommunications networks and optical interferometers, an input optical signal from a light source is typically split among multiple channels containing non-linear optical elements to drive these elements. These multiple channels then operate in parallel to provide increased capability according to the design of the systems. Such systems often suffer from errors induced by low signal levels in these optical channels as a result of the characteristics of the non-linear optical elements, e.g., relatively low conversion efficiency, and the reduction in light intensity in each channel relative to the original light intensity before its split among the channels. These limitations force system designers to increase the cost and complexity of the detector systems that receive the output signals from the non-linear optical elements.
As exemplary illustration of the shortcomings of such prior art systems, FIG. 1 schematically depicts such a prior art system having a light source 10 that emits a beam of light 12 that, for sake of illustration, is assumed to contain one unit of peak optical power. A beam splitter arrangement 14 divides the source beam 12 into multiple intermediate beams 16a, 16b, 16c, and 16d, herein collectively referred to as intermediate beams 16. Any single intermediate beam 16 contains a peak optical power that is 1/N times less than the peak optical power in the source beam 12, where N is the number of intermediate beams, here shown as four. The multiple intermediate beams 16 in turn drive multiple optical channels 18, which include non-linear elements 20, output beams 22 and optics 24 that receive the output beams 22.
Let us now examine the action within a single channel. The intermediate beam 16a contains a peak optical power that is significantly less than the peak power in the source beam 12. This can result in the non-linear optical module 20a delivering an output beam 22a to the receiving optics 24a that is significantly lessened relative to the case where the full power of the source beam 12 is supplied to the non-linear element 20a. To further illustrate this point, consider the non-linear element 20a to be a harmonic doubling crystal that can provide an output beam 22a having a peak optical power that is proportional to the square of the peak power of the intermediate beam 16a. Hence, the peak power of the output beam 22a will be (1/N)2 times lower than that obtained in an arrangement in which the undivided source beam 12 drives the non-linear element 20a. 
Further examples of such limitations of prior art systems can be found in contemporary systems for performing optical interferometry. Distance-measuring interferometers (DMI) have many industrial uses, such as positioning wafers in photolithography systems during the production of integrated circuits. For example, multiple DMI beams are typically used to measure the position and rotation angles of a stage on which a silicon wafer resides. One of the most important sources of error in DMI subsystems relates to variations in the refractive index of the air caused by atmospheric disturbances (pressure and temperature fluctuations).
A Second Harmonic Interferometer (SHI) provides a practical solution for measuring these air-induced disturbances and correcting them in real time. See, e.g., Hopf, et al., “Second-harmonic interferometers”, Optics Letters, 5:386–388, (1980). An SHI system with applications to photolithography is the subject of U.S. Pat. Nos. 5,404,222 and 5,991,033. Localized air fluctuations require monitoring in the immediate neighborhood of each DMI beam in an interferometer system that, in the case of integrated circuit fabrication, may comprise dozens of beams. SHI subsystems use a laser source to pump harmonic generation crystals at each beam location. These crystals create coherent beams at double the frequency of the incident laser light with a phase that is matched to the incident light. Frequency doubling is not efficient, and often very little second harmonic radiation is obtained, thus limiting the SHI system's signal-to-noise ratio and hence the accuracy of the measurement. Because the efficiency of a frequency doubler rises as the power density of an incident laser beam, its performance is very sensitive to the incident power. Each beam division, thus results in a quadratic diminishing of doubler output, and a resultant degradation of the signal to noise performance of the SHI. In the current state of the art, monitoring multiple interferometer beams requires many expensive lasers, or even more expensive high-power lasers and beam-splitters to divide up a single laser beam.
Hence, there is a need for an improved apparatus and method for distribution of light among a plurality of optical channels.
There is also a need for improved apparatus and method that allow efficient operation of a plurality of optical channels having non-linear optical elements while lowering of the complexity and cost associated with signal detection in such optical channels.
There is also a need for improved second harmonic interferometers and multi-wavelength fluorescence spectrometers.